(a) Field of the Invention
The present invention relates generally to data communication and, more particularly, to a method and apparatus for communication of data using a spectrally efficient modulation format.
(b) Background of the Invention
Many different modulation formats have been used for data communication, but such modulation formats typically involve communication of one, or perhaps two, symbols (each representing one or more binary digits or bits) during each signaling interval For example, one common modulation format used for data communication is binary phase shift key modulation (also known as BPSK modulation). BPSK modulation entails modulating the phase of a single carrier signal such that the modulated carrier signal has a first predetermined phase when a one-valued bit is transmitted and a second, different predetermined phase when a zero-valued bit is transmitted. Clearly, then, when using BPSK, exactly one pulse (representing a single bit) is communicated in each signaling interval, and no possibility exists for interference between the pulse representing a first bit communicated in one signaling interval and a different pulse representing a second, different bit communicated in a different signaling interval, whether adjacent in time or otherwise.
Another modulation format used to improve on the spectral efficiency of BPSK by transmitting more than one bit in each signaling interval is quadrature phase shift keying or QPSK modulation. QPSK modulation involves transmission of two bits of information during a single signaling interval. The increase in spectral efficiency of QPSK relative to BPSK is due to the resolvability of the QPSK communication carrier signal into two orthogonal (quadrature) components. Specifically, BPSK-type modulation is implemented on each of the "in-phase" and "quadrature" components of the QPSK carrier, each BPSK-type modulation using the full bandwidth of the QPSK signal. Because the in-phase and quadrature components of a carrier signal are orthogonal to one another, there is, once again, no possibility for interference between data pulses modulated onto each of the carrier components during a signaling interval, nor any possibility for interference between pulses modulated onto a carrier component in one signaling interval and other pulses modulated onto a carrier component in other signaling intervals.
These modulation formats and many others are well-known in the art of data communication. Another modulation format is described in detail in U.S. Pat. No. 4,680,777, issued Jul. 14, 1987, to Debabrata Saha and assigned to the University of Michigan, and further in Quadrature-Quadrature Phase-Shift Keying, an article by Debabrata Saha and Theodore G. Birdsall, appearing in IEEE Transactions on Communications, Vol. 37, No. 5, May 1989. Quadrature-Quadrature Phase-Shift Keying (also known as Q.sup.2 PSK modulation) is similar to QPSK modulation but entails communicating two bits of information on each quadrature component of a carrier signal during a single signaling interval. More particularly, Q.sup.2 PSK modulation encodes a bit of data on each of two, mutually orthogonal, pulses on each of the two quadrature components of a carrier signal. In that way, four bits of information can be communicated in a single signaling interval. Further, because the quadrature components of the carrier are orthogonal to one another, and the data shaping pulses modulated on to the quadrature components are also mutually orthogonal, Q.sup.2 PSK modulation would seem to offer twice the spectral efficiency of QPSK modulation with substantially the same per-bit energy requirement as QPSK.
This is not the case, however, because Q2PSK modulation employs pulses that are time-limited to a single signaling interval to avoid inter-symbol interference (ISI), and that are further mutually orthogonal to avoid cross-symbol interference (CSI). These restrictions on the number of pulses transmitted per signaling interval and on the shape of those pulses are inherent in Q2PSK modulation and operate to reduce the level of spectral efficiency that can be achieved by such modulation.
Similarly, BPSK and QPSK modulation, as well as many other well-known modulation formats, also mandate the use of only non-interfering pulses to ensure that the modulated signal can be demodulated by a receiver. As will be appreciated by those of ordinary skill in the art, this restriction to non-interfering pulses inherently and undesirably limits the level of spectral efficiency that can be attained by such modulation schemes. Further, as will also be recognized by those of ordinary skill in the art, the power required for data transmission should be minimized as much as possible for obvious reasons of cost and energy conservation. However, signals modulated by prior modulation schemes having multiple pulses per signaling interval cannot be demodulated with acceptable bit-error rates unless the signal power is elevated to an unacceptably high level.